Organic light-emitting diode (OLED) display devices utilize a current passed through thin-film layers of organic materials to generate light. Electrodes located on either side of the organic layers provide current to the organic layers. The color of the light depends on the specific organic material and the light is Lambertian and emitted in every direction. A portion of the light is emitted directly toward the front of the display device: through a substrate (for a bottom emitter device) or an encapsulating cover (for a top emitter device). A similar portion of the light is emitted toward the back of the display device and may be either absorbed or reflected by a layer behind the organic layers. If the portion of light emitted toward the back is reflected, it can pass through the organic layers again and be emitted through the front of the display, thereby increasing the brightness of the display. If the portion of the light emitted toward the back is absorbed, it is absorbed and lost.
In viewing conditions that include ambient illumination, it is important that a display device have a high contrast to improve the viewability of the display. The contrast is a ratio between light emitted from the display device and the ambient light reflected from the display device. If the contrast is very low, it is difficult for a viewer to see the display light rather than the ambient light. If the contrast is high, the display light is readily seen in comparison to the ambient light.
If the light emitted toward the back of an OLED device is absorbed, so will ambient light that enters the device. Thus, the contrast of the display is improved at the cost of half of the emitted light from the display device. If the light emitted toward the back of an OLED device is reflected, little display device light is lost but the contrast is degraded since any ambient light that enters the device is reflected back out.
Various means are known in the art to address problems with contrast. For example, circular polarizers may be placed on the front of a display device to enhance the contrast. 3M sells a circular polarizing filter HNCP37 which preferentially absorbs ambient light that passes twice through the filter rather than emitted light which passes once through the filter. Unfortunately, the polarizing filter still absorbs 60% of the emitted light, is expensive, and is exposed to the environment. Other means to provide absorbing layers by creating black layers, such as anodes or cathodes, are also known in the art. However, as described above, such an approach has the disadvantage of absorbing 50% of the emitted light.
There is a need therefore for an improved means to optimize the light output and the contrast in an OLED display device.